Carefully.
You could put the installation file on the external HDD to install it on another computer but then to install it, it would have to be on a local HDD.
When selecting the drive you wish to install the OS on, simply select the external HDD.
Yes a USB is an External HDD, but it just depends on the size....
No. After download and burn the Ubuntu 10.10 install disk, you can either TRY Ubuntu without affecting your Windows at all, or INSTALL Ubuntu to have them both in your PC. However, to install Ubuntu, you need to be careful. Make sure not to install Ubuntu to the Windows drive. That is all.
install ubuntu-restricted-extras from the package repositories or ubuntu software center.
You cannot do that. You can install the program itself on an external drive by it should be connected all the time if you want to use virus guard. If you have a virus guard which you trust on your PC you don't need to have it all your external drives. When you have connected an external drive run full scan on it.
Click the install as partition when installing Ubuntu.
Yes It Is
You don't.
PowerPC version of Ubuntu.
Yes. By booting from the Live CD and selecting "Install alongside another operating system", Ubuntu will automatically create a new partition if there is a sufficient empty space in the HDD. Otherwise, the Partitions Manager will prompt you to select the size of the new Linux partition to remove from the original OS.
When you boot from the install CD, indicate which partition you want Ubuntu installed on.